Every year numerous accidents occur when people enter unattended swimming pools, for example, after normal operating hours, and drown or are injured. These unauthorized swimmers, besides risking personal injury, may engage in vandalism and property damage. Reliable detection of these intruders can prevent personal injury and property damage, significantly reducing swimming pool operating costs.
The detection of intruders in bodies of water used for swimming is complicated by the various shapes of those bodies of water. Such a body of water may range from a small whirlpool that accommodates no more than two or three people to a very large amusement park facility and even to a natural body of water, such as a lake or a portion of a river. An intrusion detection apparatus should, preferably, be readily adaptable to these various environments without significant modification of the apparatus. In addition, an intrusion apparatus should be relatively inexpensive and relatively free of false alarms. False alarms can be caused, particularly in outdoor swimming areas, by debris falling into the body of water, strong winds, and rain. These influences can produce variations in the surface of the water that resemble a person in the water, thus resulting in false alarms. For these reasons, the state of disturbance of the surface of a body of water is an unreliable indicator of the presence of a person in the body of water. Likewise, acoustical devices, such as hydrophones, that merely listen for underwater sounds related to swimming are inherently limited in sensitivity by the movement of water, for example, through constantly operating filtration and pumping equipment in a swimming pool or by currents in a natural body of water. These sources of sound can also trigger false alarms.
Specific apparatus for detection of the presence of persons in a body of water through sensing changes in acoustical waves are disclosed in U.S. Pat. Nos. 2,783,459, 4,747,085, and 4,932,009. U.S. Pat. Nos. 2,783,459 and 4,932,009 are particularly directed to specifying not only the presence of a person in a body of water but also the location of that person within the body of water. Therefore, these systems are relatively complex and expensive. The portable apparatus described in U.S. Pat. No. 4,747,085 depends upon the establishment of a quiescent, static acoustical wave pattern within a body of water. Disturbance of the static pattern triggers an alarm, relying on the Doppler effect, i.e., a change in the frequency of a received signal as compared to the frequency of the originating acoustical signal. Such a system is susceptible to false alarms when large variations occur in the surface of the water. Moreover, the complex Doppler effect signal processing circuitry employed in the apparatus is relatively expensive.